Filtration for Borehole Installations?

In most developing countries, people rely on private water supply, mainly borehole installations as a source of drinking water. However, there is little or no treatment for borehole systems, which leaves the consumers exposed to various contaminants.

Is there a cheap and efficient filtration unit that can be installed on borehole systems to remove both chemical and microbiological contaminants? 

Are there any other inexpensive treatment methods for borehole systems?